Feedforward Control in .NET

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Feedforward Control
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feedforward system to force the process to respond as it was designedto follow the set points as directed without regard to load upsets.
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Systems for Liquid Level and Pressure
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In Chap. 3, a distinct.ion was made between those variables which are integrals of flow and those which are properties of a flowing stream. This distinction takes on added significance now, being reflected in the configuration of the feedforward system. Load is a flow term, of which liquid level and pressure are integrals. Therefore feedforward calculations for liquid level and pressure are generally linear. But where a property of the flowing stream, such as temperature or composition, is to be controlled, the system will be found nonlinear in appearance. In general, liquid-level and pressure processes appear mathematically as follows : dc 7 2 .= mK,g, - pK,g, (8.1)
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The terms K,, g,, K, and g, represent the steady-state and dynamic gain terms for the manipulated variable and load. The feedforward control system is to be designed to solve for m, substituting 1 for c:
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m = +-ldt) + c&g,
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Since dr/dt is normally zero,
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In = K,g,
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Feedforward is commonly applied to level control in a drum boiler. Because of the low time constant of the drum, level control is subject to rapid load changes. In addition, constant turbulence prevents the use of a narrow proportional band, because this would cause unacceptable variations in feedwater flow. The feedforward system simply manipulates feedwater flow to equal the rate of steam being withdrawn, since this represents the load on drum level. The system is shown in Fig. 8.2. If the two flowmeters have identical scales, which is to be expected, the ratio K,/K, of Eq. (8.2) is 1.0. Furthermore, the dynamic elements
FIG 8.2. Feedwater flow is set equal to steam flow in a drum boiler.
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g, and g, are virtually nvnexistent. solves the equation WF= W,+mL-0.5
The control system then simply
The terms WF and W, are mass flows of feedwater and steam, respectively; rnL is the output of the level controller, whose normal value is 0.5: It must be remembered that liquid-level processes such as this are nonself-regulating. The controlled variable will consequently drift unless feedback is applied. Since integral feedback may not be used alone, because instability would result, a two-mode controller is always used. In the steady state, feedwater flow will always equal steam flow, so the output of the level controller will seek the bias applied to the computation. If the controller is to be operated at about 50 percent output, that bias must be 0.5, as indicated in the formula. The controller does not have to integrate its output to the entire extent of the load change with a forward loop in service, but need only trim out the change in error of the computation during that interval. This feedforward system has two principal advantages: 1. Feedwater flow does not change faster or farther than steam flow. 2. Control of liquid level does not hinge upon tight settings of the feedback controller. Because this feedforward system, like many, is based on a material balance, accurate manipulation of feedwater flow is paramount. In general, the output of a feedforward system is the set point for a cascade flow loop and does not go directly to a valve. Valve position is not a sufficiently accurate representation of flow.
Systems for Temperature and Composition
Temperature and composition are both properties of a flowing stream. Heat and material balances involve multiplication of these variables by flow, producing a characteristic nonlinear process model. Feedforward systems for control of these variabIes are similarly characterized by multiplication and division. The general form of process model for these applications is
A single coefficient K, is sufficient to identify the steady-state gain. The feedforward equation to control this general process is simply the solution for m, replacing c with r:
%q m=K,g,
(8.4)
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